The invention relates to a method for forming a paper web by which web formation is improved and flocculation is reduced.
The invention further relates to a twin-wire former of a paper machine including a carrying wire and a covering wire whose loops are guided by forming rolls and guide rolls to form a twin-wire forming zone at the region of the forming rolls, the forming zone beginning at the forming gap defined by the wires and into which gap a pulp suspension jet is fed from the slice of a head box.
The invention also relates to a forming roll for a paper machine which includes a perforated mantle within the interior of which a chamber system is arranged which includes at least one negative-pressure chamber which communicates with at least one negative-pressure zone defined by sealing members which act against the inner surface of the roll mantle.
In old single-wire forming machines, vibrating forming boards were used in order to improve the formation of the web being formed. Although this technique has proved useful in connection with single-wire machines, it is not suitable for use in modern high-speed paper machines and, in particular, in twin-wire formers. One technique used to improve web formation is by appropriately shaping the suspension ducts of the head box, discussed below.
As is known, attempts have been made to improve formation of the paper web in twin-wire formers by providing various combinations of forming members, such as forming rolls and stationary forming shoes, and by selecting the dewatering directions of the web so that by means of, for example, lath shoes of the stationary forming shoes, a pulsating dewatering pressure is produced in the pulp layer being formed. The pulsating dewatering pressure affects the homogeniety of the fiber structure in the web to reduce its flocculation. However, it is frequently not possible by these methods to alter the non-homogeniety of the pulp suspension in the jet being discharged from the head box slice sufficiently efficiently. Improvement in web formation obtained by shaping the suspension ducts in the head box is also reaching a limit as illustrated by the following example. The output of the feed pump for the head box is typically 1.2 MW. Nevertheless, the microturbulence output of the head box, produced by means of the turbulence generator of the head box, is only on the order of about 20 kW. The microturbulence output of the pulp suspension flow immediately after the turbulence generator is on the order of about 1.2 kW and the output remaining at the discharged jet is only about 5 W.
The homogeneity of the formed paper web has become even more important in recent years with the introduction of thinner printing paper qualities and with the advent of complicated high speed printing machines. On the other hand, the homogeneity of the paper web which is produced also affects the running quality of the paper machine in that it is usually the weakest portions of the web, i.e., the non-homogeneous portions thereof, that cause breaks in the paper web and consequent down time of the machine.
It is also known to use steam implosion in the former of a paper machine to improve the formation of the web and to reduce web flocculation. In this respect, reference is made to U.S. Pat. No. 3,981,768, assigned to applicant's assignee, as well as to U.S. Pat. Nos. 3,970,513 and 3,992,254.